Abell’s richest cluster

The Universe contains some truly massive objects. Although we are still unsure how such gigantic things come to be, the current leading theory is known as hierarchical clustering , whereby small clumps of matter collide and merge to grow ever larger. The 14-billion-year history of the Universe has seen the formation of some enormous cosmic structures, including galaxy groups , clusters , and superclusters — the largest known structures in the cosmos!

This particular cluster is called Abell 665. It was named after its discoverer, George O. Abell , who included it in his seminal 1958 cluster catalogue . Abell 665 is located in the well-known northern constellation of Ursa Major (The Great Bear). This incredible image combines visible and infrared light gathered by the NASA/ESA Hubble Space Telescope using two of its cameras: the Advanced Camera for Surveys and the Wide Field Camera 3 .

Abell 665 is the only galaxy cluster in Abell’s entire catalogue to be given a richness class of 5, indicating that the cluster contains at least 300 individual galaxies. Because of this richness, the cluster has been studied extensively at all wavelengths , resulting in a number of fascinating discoveries — among other research, Abell 665 has been found to host a giant radio halo , powerful shockwaves , and has been used to calculate an updated value for the Hubble constant (a measure of how fast the Universe is expanding).